It is well known to determine signal quality parameters of electrical or optical networks, including cable networks such as CATV (Cable television) networks. Parameters or measures indicative of the signal quality at the receiving end (typically the subscriber end) of the network are, for example, the signal-to-noise ratio (SNR) and, in digital networks, the bit error ratio (BER). By determining the noise level (and/or another property) of the output signal, an indication of the signal quality at the receiving end of the network can be obtained.
A cable network contains cables, amplifiers and other components. Cables or wires have relatively little influence on the signal quality, but they attenuate high frequency components. Amplifiers typically introduce noise. It will be understood that the noise of a number of amplifiers arranged in series, as used in typical cable networks, accumulates and may affect the actual signal.
In addition, amplifiers introduce signal distortion due to non-linearities. Ideally, an amplifier outputs the input signal s(t)in multiplied by a gain factor:s(t)out=A·s(t)in where the gain A is constant. In practice, however, amplifiers are not perfectly linear and the output signal will typically contain higher powers of the input signal, including quadratic and cubic terms:s(t)out=A·s(t)in+B·s(t)in2+C·s(t)in3+ . . .As a result of this non-linearity, so-called intermodulations of input signal frequencies will cause the output signal to contain frequency components that were not present in the input signal. These undesired intermodulations lower the signal quality and should therefore be taken into consideration when determining the signal quality of a network.
However, conventional methods typically fail to take these intermodulations into account. Even when intermodulations are taken into account, they are typically lumped together, thus failing to accurately determine their individual contributions to the overall signal quality level.
The paper “Frequency Response of Nonlinear Networks using Curve Tracing Algorithm” by A. Yoshida, Y. Yamagami & Y. Nishio, IEEE, May 2002, discloses a method of calculating the characteristic curves of non-linear circuits. Although non-linear effects are taken into account, only the fundamental frequency components are taken into account. Accordingly, the impact of intermodulation on only a single frequency is considered, making the known method unsuitable for determining the intermodulation effects of frequency bands. In addition, said Prior Art paper fails to refer to cable networks.